The present invention relates generally to a liquid degassing apparatus, and more particularly to a degassing apparatus in the ink supply system of a thermal ink jet printing device.
A thermal ink jet printer has at least one printhead in which thermal energy pulses are used to produce vapor bubbles in ink-filled channels and so cause droplets of ink to be expelled from the channel orifices towards a recording medium. The thermal energy pulses are usually produced by resistors, each located in a respective one of the channels, which are individually addressable by current pulses to heat and vaporize ink in the channels. As a vapor bubble grows in any one of the channels, ink bulges from the channel orifice until the current pulse has ceased and the bubble begins to collapse. At that stage, the ink within the channel retracts and separates form the bulging ink which forms a droplet moving in a direction away from the channel and towards the recording medium. The channel is then refilled by capillary action, which in turn draws ink from a supply container. It is conventional to provide an arrangement to clean the channel orifices periodically while the printhead is in use and to close-off the orifices when the printhead is idle to prevent ink in the printhead from drying out.
One form of thermal ink jet printer is described in U.S. Pat. No. 4,638,337 to Torpey et al. That printer is of the carriage type and has a plurality of printheads, each with its own ink supply cartridge, mounted on a reciprocating carriage. The channel orifices in each printhead are aligned perpendicular to the line of movement of the carriage and a swath of information is printed on the stationary recording medium as the carriage is moved in one direction. The recording medium is then stepped, perpendicular to the line of carriage movement, by a distance equal to the width of the printed swath and the carriage is then moved in the reverse direction to print another swath of information. As an alternative to providing each printhead with its own ink cartridge, the printheads can be supplied with ink from one or more supply tanks which need not be mounted on the carriage.
U.S. Pat. No. 4,454,518 refers to the importance of temperature control in an ink jet printer and, in particular, the control of the ink temperature in a printer of the type that utilizes a piezoelectric transducer to cause the discharge of ink droplets from a printhead. U.S. Pat. No. 4,929,063 describes the cooling of the printhead of a thermal inkjet printer by causing ink to flow through the printhead in a volume far greater than that required for printing purposes. Temperature control of thermal inkjet printheads is also discussed in U.S. Pat. Nos. 4,896,172 and 4,980,702.
U.S. Pat. No. 5,121,130 to Hempel et al. discloses a printhead assembly for a thermal ink jet printer in which the ink supply path carrying ink to the printhead passes through, and receives heat form, a heat sink adjacent the print heaters of the printhead. The ink then passes to the printhead via a secondary reservoir, the position of which relative to the printhead establishes the ink pressure at the printhead discharge orifices. Capping means is provided to cap the discharge orifices when the printhead is idle and to purge ink from the printhead when required.
Problems arise with ink jet printers which are known in the art due to dissolved gases being present in the ink in the ink reservoir. As well as making it difficult to control the temperature of the ink, the dissolved gases can have a significant effect on the amount of ink expelled in a droplet when the ink near a channel orifice is heated, and on the manner of its expulsion, thereby reducing print quality. When a heater resistor near a channel orifice is addressed, dissolved gases in the vicinity expand and even merge with the ink vapor bubble, thereby distorting the vapor bubble, and therefore the ink droplet, from its optimum volume and shape. Also, in ink jet printers it is conventional to maintain a negative pressure at the printhead; the presence of air bubbles may make this difficult. A degree of resistance to flow of the ink may also be produced by dissolved gas bubbles, particularly when collected around filter screens so that capillary refill is impeded or blocked.
The present invention seeks to provide a liquid degassing apparatus for removing gases dissolved in a liquid. The present invention further seeks to provide an ink degassing system in the ink supply system of an ink jet printer. The present invention further seeks to provide ink supply system of an ink jet printer in which the ink temperature and the amount of dissolved gas in the ink is precisely controlled.